The invention relates to a mask set containing at least two masks coordinated with one another for projection onto the same surface. A first mask has transparent first regions and second regions, which phase-shift in relation thereto for producing dimensionally critical structures, and a second mask has transparent regions and opaque regions for producing further structures. The invention further relates to a process for lithographically structuring a surface. In the process the two masks are projected onto the surface, the transparent first regions and second regions, which phase-shift in relation thereto, being disposed on the first mask in order to produce dimensionally critical structures, and the transparent regions and opaque regions disposed on a second mask for producing further structures.
During the lithographic structuring of resist layers which is carried out in semiconductor fabrication, use is made of phase masks with which, through the use of phase contrasts, structures whose dimensions are of the same order of magnitude or even smaller than the exposure wavelength used, are exposed. The phase contrasts are, for example, created by height steps between mask fields, i.e. by different mask thicknesses. When such masks are projected, shadow lines with a large process window, i.e. with a large tolerance relative to variations in the exposure intensity or in the projection focusing are formed by destructive interference below the height steps, which usually produce a phase change of half the exposure wavelength. The destructive interference at the height steps causes the formation of phase-contrast lines with a width below the exposure wavelength. With such a phase mask, it is possible to produce the smallest dimensionally critical structures.
Since height steps only occur between mask fields of normal thickness and mask fields of increased or reduced thickness, the phase-contrast lines projected by phase masks are always closed on themselves. When more complex patterns with branch line structures and line ends need to be produced, use is made of so-called trimming masks which are projected in addition to the first mask onto the surface to be exposed and resolve the phase conflicts due to the structure. The known trimming masks having optically transparent regions that are bounded by a chromium layer and are projected onto the already exposed phase-contrast lines and structure them further. With the aid of the chromium layer, it is further possible to expose other structures of not so small and therefore noncritical dimensions onto the surface. In this way, with the aid of two masks, the smallest dimensionally critical structures can be projected in addition to larger and therefore less critical structures.
However, this technique has the disadvantage that the structured phase-contrast lines, that is to say interrupted in the length direction, are separated from one another by distances that are large compared with the line width, because the chromium mask used for the trimming cannot produce such fine structures as the phase mask.
Moreover, it is not possible for the closed phase-contrast lines exposed using the phase mask to be structured with the aid of a further phase mask, because the latter would re-expose the unexposed phase-contrast lines, that is to say shadow lines, everywhere except the crossover points, and would therefore eliminate them.
It is accordingly an object of the invention to provide a trimming mask with semitransparent phase-shifting regions that overcomes the disadvantages of the prior art devices and methods of this general type, in which fine structuring phase-contrast lines which are very fine in terms of their width along the line profiles are possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a mask set, containing at least two masks, including a first mask and a second mask, coordinated with one another for projection onto a same surface. The first mask has transparent first regions and second regions which phase-shift in relation thereto for producing dimensionally critical structures. The second mask has transparent regions and opaque regions for producing further structures, and the second mask further has phase-shifting regions.
In accordance with an added feature of the invention, the phase-shifting regions of the second mask are disposed for further structuring of the dimensionally critical structures produced by the first mask.
The object is achieved, in relation to the mask set, in that the second mask has the phase-shifting regions in addition to the transparent and opaque regions. Through the addition of the phase-shifting regions, especially fine structural elements for structuring the phase-contrast lines produced during the first exposure can be produced with the aid of phase contrast. The line sections formed in this way can follow one another substantially more closely than if a merely chromium-covered trimming mask is used. In a preferred embodiment, the semitransparent phase-shifting regions of the second mask are disposed in such a way that they further structure the dimensionally critical structures produced by the first mask. Preferably, the second mask has isolated transparent and/or opaque regions with a semitransparent phase-shifting edge. Preferably, some of the transparent regions of the second mask, which are provided with a semitransparent phase-shifting edge, are used to interrupt the phase-contrast lines produced using the first mask. To that end, wherever an interruption is intended, they are disposed exactly over the exposed phase-change lines.
In a refinement of the invention, the second mask has isolated phase-shifting areas. Using these, it is possible to produce just as fine phase-contrast lines as with the first mask, although they cannot then be structured further. The isolated transparent or opaque regions with a phase-shifting edge may likewise be used to produce further structures.
The first mask need not necessarily contain only phase structures, but may have opaque regions just like the second mask. It is expedient if the phase-shifting regions of the first mask are etched more deeply or less deeply than the regions that are not phase-shifting. The phase-shifting regions of the second mask are preferably semitransparent, that is to say they transmit between 5 and 10, preferably 6% of the incident radiation. It is expedient if the phase change produced by the phase-shifting regions is approximately 180xc2x0, and therefore corresponds to a half wavelength.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a process for lithographically structuring a surface. The process includes projecting at least two masks, including a first mask and a second mask, onto the surface. The first mask has transparent first regions and second regions which phase-shift in relation thereto for producing dimensionally critical structures. The second mask has transparent regions and opaque regions for producing further structures and the second mask further has phase-shifting regions.
In terms of the process, the object is achieved in that phase-shifting regions are also disposed on the second mask in addition to the transparent and/or opaque regions. The phase-shifting regions bring about an increase in the intensity gradients of the projected structures. This allows sharper projection of smaller structures than with the conventional chromium mask directed at structures of substantially larger dimension.
In one advantageous implementation, the phase-shifting regions of the second mask are disposed and projected in such a way that they further structure the dimensionally critical structures produced by the first mask. In this case, those regions of the phase-contrast line which are intended to undergo no further alteration by the second mask are protected by its chromium layer against re-exposure, so that erosion of the latent image of the first mask is avoided; the image contrast produced is preserved during the second exposure.
Advantageously, the closed phase-contrast lines are interrupted by the projection of the second mask, by locally re-exposing them through transparent regions of the second mask with semitransparent phase-shifting edges. The individual line sections can be disposed significantly closer together than when an opaque chromium mask is used, without substantially compromising the process window.
In accordance with a concomitant feature of the invention, there is the step of locally re-exposing the phase-contrast lines through the transparent regions of the second mask with semitransparent phase-shifting edges.
Lastly, spans of a dimensionally critical width are produced in a positive resist.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a trimming mask with semitransparent phase-shifting regions, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.